1. Field of the Invention
The present invention relates to a motor control apparatus for a sewing machine, more particularly to the motor control apparatus for stopping a needle of the sewing machine by using an inverter.
2. Prior Art
Motors for controlling a needle of a sewing machine, such as direct current servo motors and clutch motors have been widely used. Maintenance is necessary for the direct current servo motor disclosed in the U.S. Pat. No. 4,137,860 since it has a brush. A clutch motor disclosed in the U.S. Pat. No. 3,910,211 needs a brake mechanism and needs to activate the motor while stopping the needle so that power loss and poor durability derive therefrom. Recently, there has been proposed a control apparatus disclosed in U.S. Pat. No. 4,627,370 for controlling a speed of an induction motor by an inverter.
In this inverter, the motor is decelerated in accordance with regenerative braking, direct current braking or negative-phase braking, and so on. Control of the stop position of the needle, however, is done by a mechanical electromagnetic brake so as to stop the needle at the predetermined position in upward and downward direction. Accordingly, the electromagnetic brake should be incorporated into the motor, which enlarges the motor. Also the drive circuit of the electromagnetic brake as well as the inverter circuit is needed, adding to apparatus's high cost.
Furthermore, a synchronous motor disclosed in EPC Appln. No. 176599 using the inverter as motor controller of the sewing machine has been proposed and developed. In this motor, however, a permanent magnet is generally attached to a rotor and there is a need to provide a detector for detecting a rotational position of a rotor so as to change the energization of stator coils. Accordingly, the motor configuration gets complicated, adding to high cost.